The present invention relates to heat exchange media in regenerative thermal oxidizers (RTOs). More particularly, the invention relates to heat exchange media for use in heat exchangers in RTOs, and the resulting improved thermal oxidizers.
Regenerative thermal oxidizers are preferably used for destroying volatile organic compounds (VOCs) in high flow, low concentration emissions from industrial and power plants. RTOs typically require high oxidation temperatures in order to achieve high VOC destruction and high heat recovery efficiency. To more efficiently attain these characteristics, the "dirty" process gas which is to be treated is preheated before oxidation. A heat exchanger column is typically provided to preheat these gases. The column is usually packed with a heat exchange material having good thermal and mechanical stability and high thermal mass. In operation, the process gas is fed through a previously heated heat exchanger column, which, in turn, heats the process gas to a temperature approaching or attaining its VOC oxidation temperature. This pre-heated process gas is then directed into a combustion chamber where VOC oxidation is usually completed.
The treated "clean" gas is then directed out of the combustion chamber and back through the heat exchanger column, or, according to a more efficient process, through a second heat exchange column. As the hot oxidized gas is fed through the column, the gas transfers its heat to the heat exchange media in the column, cooling the gas and pre-heating the heat exchange media so that another batch of process gas may be preheated prior to the oxidation treatment. Usually, an RTO has at least two heat exchanger columns which alternately receive process and treated gases. This process is continuously carried out, allowing a large volume of process gas to be efficiently treated.
The performance of an RTO may be optimized by increasing VOC destruction efficiency and by reducing operating and capital costs. The art of increasing VOC destruction efficiency has been addressed in the literature using, for example, means such as improved oxidation systems and purge systems. Operating costs can be reduced by increasing the heat recovery efficiency, and by reducing the pressure drop across the oxidizer. Operating and capital costs may be reduced by properly designing the RTO and by selecting appropriate heat transfer packing materials. While design aspects of RTOs have been the subject of prior patent literature, the choice of the heat transfer packing material has not been sufficiently addressed.
It is therefore an object of the present invention to provide an arrangement of heat exchange media which provides a significant high heat recovery by low pressure drop of an RTO, thereby reducing costs associated with the process.
The properties of a bed of packing material, such as the shape, size, and packing characteristics, determine the heat recovery, pressure drop, and cycle time of the RTO. For example, heat recovery is proportional to the heat transfer coefficient and the heat capacity per unit bed volume. Cycle time, like heat recovery, is proportional to the bed heat capacity per bed column. For a given packing material, heat capacity per bed volume is inversely proportional to bed void fraction. Pressure drop is also inversely proportional to the bed void fraction. Thus, in conventional bed packing materials, a higher bed void fraction decreases not only pressure drop (which reduces operating cost) but also decreases heat recovery and cycle time (which increases operating cost).
In order to obviate the problems associated with pressure drop, monolith structures have been proposed (see, e.g., U.S. Pat. No. 5,352,115 to Klobucar). Such structures, however, suffer from a decreased heat recovery. Further, the continuous structure of the Klobucar material renders it vulnerable to thermal stresses due to a thermal gradient from the inlet portion of the monolith to the outlet portion of the monolith. These stresses may cause cracking and premature failure of the monoliths resulting in costly, unscheduled downtime of the RTO and replacement of the monoliths. Monolith structures are also expensive.
A number of different shapes of packing materials have been disclosed in the prior art. As disclosed below, however, shapes have primarily been used as contacting or mixing devices and as catalyst pellets. The influence of the size and shape of the heat transfer material on the heat transfer, heat storage, and pressure drop in RTOs has not been discussed.
For example, U.S. Pat. No. 3,907,710 (Lundsager) discloses a four- and six-ribbed wagon wheel-shaped material. This material has been proposed as a contacting device in a packed tower or column or as an inert support on which catalytic ingredients may be deposited to perform catalytic reactions.
U.S. Pat. No. 4,610,263 (Pereira et al.) discloses an extrudate suitable for improved gas-liquid contacting which is made from a solid transitional alumina. This material has a similar shape to one of the materials disclosed in the present invention. The cylindrical extrudate has partially hollow interior and internal reinforcing wings extending from the inner wall to the center of the extrudate particle. The transitional alumina of the reference has a BET nitrogen surface area of at least 50 m.sup.2 /g, a diameter of up to about 6.5 mm, an aspect ratio of length to diameter of from 0.5 to 5, a geometric surface area of at least 25% greater than a hollow tube of the same inside and outside diameter, a porosity of at least 0.3 cm.sup.3 /g, and a surface area per reactor volume of at least 5 cm.sup.2 /cm.sup.3.
In light of the foregoing, there is a need for low-cost and simple heat exchange media having excellent thermal and mechanical stability and high thermal mass which will not exhibit a large pressure drop when packed into heat exchange column for an RTO.